Field of the Invention
The present invention relates to a honeycomb structure, and more particularly, it relates to a honeycomb structure which is for use in an exhaust gas purifying device or the like to trap particulate matter and purify and treat an exhaust gas, which simultaneously exerts both of a high temperature raising performance and a high heat capacity, and which is capable of efficiently perform a purifying treatment of the exhaust gas or the like.
Description of the Related Art
In various fields of cars, chemistry, electric power, iron and steel, and others, a ceramic honeycomb structure which is excellent in heat resistance and corrosion resistance has been employed as a carrier in a catalyst device for use in an environmental measure, collection of specific substances or the like, or as a filter. For example, with reinforcement of regulation on an exhaust gas from a diesel engine or a direct injection type gasoline engine, there is used an exhaust gas purifying device or the like in which a diesel particulate filter (DPF) or a gasoline particulate filter (GPF) using the above honeycomb structure is employed to trap particulate matter (PM) included in the exhaust gas. As a material of the honeycomb structure for use under an atmosphere of a corrosive gas at a high temperature, there is especially suitably used a ceramic material such as silicon carbide (SiC), cordierite or aluminum titanate (AT) which is excellent in heat resistance and chemical stability.
In the DPF or the like, cell surfaces of the honeycomb structure are coated with a catalyst to oxidize and purify the particulate matter. Here, for the purpose of efficiently burning the particulate matter deposited in cells and achieving regeneration of the DPF or the like, it is necessary to immediately raise a temperature of the honeycomb structure up to a temperature to activate the catalyst (an activation temperature), and it is necessary to maintain the activation temperature for a long time. Consequently, it is possible to stabilize a purification efficiency of the DPF or the like which traps the particulate matter and performs a purifying treatment by the catalyst.
However, the exhaust gas emitted from the diesel engine might not reach the activation temperature of the catalyst, in a case of an operation at a comparatively low exhaust temperature and further in a low load state. Furthermore, also in the gasoline engine, it is necessary to more immediately raise the temperature up to the activation temperature of the catalyst even at start of the engine in a case of an operation of the engine in the low load state. On the other hand, also in a case of an operation of the engine in a high load state, when the state rapidly shifts to the low load state, the temperature of the honeycomb structure immediately lowers to be not more than the activation temperature of the catalyst. That is, the purification efficiency might deteriorate because a heat capacity of the honeycomb structure is low.
To eliminate such a problem, for the purpose of improving a temperature raising performance, a partition wall thickness of partition walls constituting the honeycomb structure is decreased or a porosity is increased. However, the above technique has the problem that the heat capacity of the honeycomb structure further decreases. Furthermore, the decrease of the partition wall thickness has the problem that a mechanical strength of the honeycomb structure remarkably decreases. As a result, in a canning operation of canning and attaching the honeycomb structure into a can member, a strong external force is applied to the honeycomb structure, thereby causing the fear that cracks and the like are easily generated in the partition walls. Consequently, there increases the possibility that the honeycomb structure breaks.
On the other hand, in a case where the honeycomb structure is constituted of the partition walls using a material having a high heat capacity, the problem that the temperature lowers rapidly from a high temperature time is eliminated, but there is the fear that the temperature raising performance to reach the activation temperature from the start of the diesel engine or the like remarkably deteriorates. That is, the high temperature raising performance and the high heat capacity are contradictory to each other.
Thus, for the purpose of improving the strength of the honeycomb structure and eliminating disadvantages in the canning operation, it has already been suggested that a partition wall thickness of partition walls defining cells of a honeycomb structure is suitably changed to arrange regions having large and small partition wall thicknesses in one honeycomb structure.
For example, there has been suggested that a thickness of partition walls defining cells positioned in the vicinity of a circumferential wall of a honeycomb structure is adjusted to be larger than that of partition walls defining cells positioned in another region (see Patent Document 1). Furthermore, for the purpose of avoiding erosion of a part of the honeycomb structure which comes in contact with a high-temperature exhaust gas, there has been suggested a honeycomb structure in which a partition wall thickness of partition walls of a region directly hit by the exhaust gas is increased (see Patent Document 2). Consequently, it is possible to improve a strength of a part of the honeycomb structure, and it is possible to eliminate disadvantages especially in a canning operation.
Alternatively, there has been suggested a honeycomb structure in which in partition walls defining quadrangular cells, there is increased a partition wall thickness of the respective partition walls formed along one axis (e.g., an X-axis) in axial directions (the X-axis and a Y-axis) of the partition walls (see Patent Document 3), and there has been suggested a honeycomb structure in which in partition walls defining quadrangular cells, the partition walls having two types of partition wall thicknesses are uniformly arranged linearly (see Patent Document 4).    [Patent Document 1] JP-A-2002-326034    [Patent Document 2] JP-A-2002-326035    [Patent Document 3] JP-A-2003-181233    [Patent Document 4] JP-A-2010-234315